Glycans have several distinct properties that make their development as disease biomarkers appealing. Firstly, their location on cell surfaces makes them the first point of contact for cellular interactions, and thus they are crucial in the control of normal metabolic processes, and conversely, they function as pathogen adhesion receptors. Secondly, specific glycan structures that are not present, or are in low amounts in normal state, proliferate or alter their sequence in disease states. And, lastly, changes in glycosylation may be found in many proteins, including those that are highly abundant. Thus changes in the normal levels of glycan structures, such as terminal sialic acid, may be markers of disease states. New highly-specific reagents are required in order to overcome current limitations in the discovery and exploitation of disease-related glycans. Using structurally-guided genetic manipulations, we are converting the NanB sialidase from S. pneumococcus into a high-specificity affinity reagent for the detection of all types of sialic acid modifications of glycopeptides and glycoproteins. Because such a protein has lectin-like properties, but is derived from an enzyme, it is called a Lectenz. A NanB Lectenz addresses a key need in disease glycomarker detection: namely, a robust and easy to produce reagent specific for detecting all types of sialylated glycans. This reagent could be employed in an affinity matrix for sample enrichment, which in conjunction with existing MS based methods could provide linkage information. Glycopeptide sample enrichment aids glycomic analyses by eliminating non-glycosylated peptides, which would otherwise attenuate the signals from glycopeptides that have low ionization efficiency. Glycosylation site mapping is essential in fully characterizing and exploiting glycans as markers of specific disease states, but at present, no reagent exists that can detect sialylated glycans, independent of the type of linkage associated with the sialic acid. Thus, at present, a number of reagents with varying specificities and affinities must be employed to capture or detect all forms of sialylated glycans. Lectenz offer numerous advantages over plant lectins: they are engineered to be high affinity and yet retain the exquisite substrate specificity of the endogenous enzyme, they may be efficiently produced, and for human homologues have the potential to be employed in vivo with low toxicity. Whereas some aspects of Lectenz development parallel those of antibody evolution, Lectenz have the tremendous benefit of employing a protein nave template that has the desired specificity.
Using structurally-guided genetic manipulations, we are converting carbohydrate-processing enzymes into a high-specificity affinity reagents (called Lectenz) for the detection of disease-related modifications of glycopeptides and glycoproteins. Here we are developing a Lectenz (Sia-PS1) specific for the enrichment of biologically-important glycans that contain all forms of sialic acid. The principle advantages of engineered Lectenz over other reagents, such as antibodies or lectins, is that they have exquisite substrate specificity that is not context dependent, they may be evolved to have desirable binding kinetics, and that they may be efficiently produced as monomeric proteins.
